A technician is troubleshooting a laptop that sometimes fails to charge.

When a laptop sometimes fails to charge despite a good power adapter connection, the most likely issue is a defective DC jack. The DC jack, where the power adapter plugs into the laptop, can become loose or damaged over time, leading to intermittent or failed charging. Cooling fan issues, bad memory, or an overheating CPU would typically lead to different symptoms such as overheating, system crashes, or performance issues, not charging problems.